Energy pellet

ABSTRACT

A high-energy water-resistant pellet of at least 75% biomass material such as torrefied wood, whole-tree (white) wood, agricultural waste, flax and the like. and the remainder a binder comprising from about 2% to about 20% by total weight of the pellet. The binder is a two-component system—a plasticizer, such as tall oil pitch, rosin, fatty acid, vegetable oils, animal oils, corn protein and glycerin—preferably from 3% to about 20% of the binder, and lignin from about 80-97% of the binder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a continuation in part of application Se. No.14/414,202 filed Jan. 12, 2015, which claims benefit of PCT ApplicationU.S. 2013/050679, filed Jul. 16, 2013, which claims benefit ofProvisional Application Ser. No. 61/673,573 filed Jul. 19, 2012, onwhich the present Application is based and benefit claimed under 35U.S.C. §119(e), is herein incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to energy pellets made from biomassmaterial, such as torrefied wood, whole-tree (white) wood, agriculturalwaste, flax and the like. It also relates to methods of making energypellets. More particularly, this invention relates to energy pelletsmade from torrefied wood or white wood combined with lignin and abiomass-based plastizer to form a water-resistant and durablehigh-energy-density pellet.

Description of Related Art

In recent years renewal energy sources have become more desirable andthus more important although the United States continues to be heavilyreliant on the combustion of non-renewable fossil fuels to meet itsenergy needs. Additionally, energy from biomass mandates in the EuropeanUnion has led to a rapidly growing export market for energy pellets.Exports of white-wood pellets to the EU from North America was 4.4million metric tons in 2012 and is predicted to reach 25-70 millionmetric tons per year by 2020. To meet the growing energy needs biomassbased materials have been pelletized to provide a dense, relatively highenergy material.

One of the desirable biomass materials being developed is torrefiedwood. Torrefied wood is wood that has been partially oxidized to driveoff the water and the organic volatiles and a fraction of the cellulose,so that a “blackened” wood chip is the result. The torrefied wood chipsare friable and can be easily formed into a high-energy, high-densitypellet. When a pellet is made from southern yellow pine wood with itshigh resin content, it will remain as a pellet for days when immersed inwater—indicating excellent water repellency. A pellet made from southernyellow pine needs no binder, since the pellets from that wood is exposedto the 40,000+ psi of the extruder and temperatures exceeding 100° C.,the natural components flow to form a water repellant binder naturally.However, when a pellet is made from torrefied wood, most of the naturalbinders are burned off so that the resulting pellet has poor waterrepellency, and in a worst case, the pellets turn into a black “mud”upon exposure to water.

Another problem with torrefied wood and even with some of the otherbiomass materials, especially those made from low-resin-containingbiomass materials, is durability. The pellets themselves tend to befriable and create dust during mechanical processes needed to transportthe pellets, such as loading and unloading rail cars and ships. As muchas 5-7% loss of mass from dust has been reported for white-wood pellets.In addition to product loss, the dust creates hazards, from inhalationand the potential for dust explosions.

Torrefied wood has the strong advantage of being closer to coal in itsburning capabilities than white wood. And the energy (bulk) density oftorrefied wood is much greater than white wood so its shipping costs aremuch lower.

The high-energy, high-density and coal-like mechanical properties oftorrefied wood pellets are desirable for large biomass burningfacilities—like the utilities but the coal is normally transported inuncovered cars to and from ports and stored in uncovered piles at thecombustion site, so water repellency is a key attribute. A singleutility would have to spend $100 million or more to cover these pelletpiles and rail cars, and few if any will make this capital expenditure,so water repellency is strongly desirable.

Currently wood-based energy pellets are burned, but the lower energydensity and their pulverizing and burning characteristics limit theiruse as a fuel since they are so different from coal. Energy pellets madeentirely of lignin has approximately the same energy content as coal,about 12,000 Btu/lb, which is about 50% higher energy per mass oflow-moisture wood pellets having about 8,000 Btu/lb.

Within the wood matrix, lignin is a macromolecule is chemically bound tocellulose and hemicellulose. In pulping processes or enzymaticbiorefineries, the lignin is separated from the cellulose andhemicellulose and its molecular weight is reduced (i.e., the lignin is“chopped up”). This isolated lignin at ambient conditions is a solid,but it can be melted at elevated temperature, with lower molecularweight lignin generally having a lower melt point. The problem is thatisolated solidified lignin is very friable, and its melt point may notbe ideal for the pelletizing system so that the lignin melts and flowswithin the pelletizer but solidifies upon exiting and cooling.

The lignin content in wood has recently been shown (Thomas Wilson, PhDThesis, Penn State University, 2010) to have no effect on the durabilityof white wood pellets. In the Penn State study, a wide variety of woodwith varying lignin contents was used; however, all that lignin was inthe macromolecular form and chemically bound to cellulose. Contrary tothe Penn State study, lignin with lower molecular weight that has beenisolated from wood has better binding capability. And the macromolecularlignin used in the Penn State study may not have the required melt-pointprofile.

For a binder to work well in forming energy pellets, the binder mustmelt and flow under the extruder conditions of elevated temperature andpressure. As the pellets emerge from the extruder and cool, the bindermust reform as a solid, coating and protecting the smallerelements—loose fibers of white wood or particles of torrefied wood.

Lignosulfonate is a byproduct of the sulfite pulping process.Lignosulfonates have demonstrated commercial utility as binders foranimal foods. Lignosulfonates are a poor choice as energy pellet bindersbecause: (1) lignosulfonates have high levels of organically-boundsulfur—as high as 10% by mass—that converts to sulfur dioxide uponcombustion; and (2) lignosulfonates contain residual sugar from thepulping process. These sugars are hydrophilic and deleteriously affectthe water resistance of energy pellets. The sulfonate groups on thelignin are also hydrophilic, exacerbating the water resistance problem.

SUMMARY OF THE INVENTION

There is provided a high-energy water-resistant pellet having at least75% biomass material, such as torrefied wood, whole-tree (white) wood,agricultural waste, flax and the like, and the remainder being atwo-component binder comprising from about 2% to about 25% by totalweight of the pellet. If the starting material is torrefied wood it willhave a moisture content of below 15% water, with the equilibria watercontent being about 10-15% water. The binder is a two-component systemcomprising a plastizer and lignin. The plastizers used in the binder aretotally natural organics, preferably tall oil pitch, fatty acids, rosinglycerin and the like. The plastizer serves to adjust the melt point,preferably from 3% to about 20% of the binder, and lignin from about80-97% of the binder.

There is also provided at method for making a high-energywater-resistant pellet from biomass material and a two component binder.

It is therefore the general object of the present invention to provide ahigh-energy, high density fuel pellet made from torrefied wood or whitewood or agricultural waste or flax and a binder that is water repellent.

Another object of the present invention is to provide a high-energy,high-density water-repellant fuel pellet from flax and a two-componentbinder.

Yet another object of the present invention is to provide a process forproducing a high-energy pellet.

Other objects, features and advantages of the invention will be apparentto those skilled in the art from the following detailed description ofthe preferred embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter inwhich preferred embodiments of the invention are shown. This inventionmay, however, be embodied in many different forms and should not beconstrued as limited to the embodiments set forth herein; rather theseembodiments are provided so that this disclosure will be through andcomplete and will fully convey the scope of the invention to thoseskilled in the art.

The energy pellets may be made from biomass material, such as torrefiedwood, whole-tree (white) wood, agricultural waste, flax and the like. Apreferred fuel pellet of the present invention is produced fromtorrefied wood having a moisture content of below 15% water, with theequilibria water content being about 10-15% water, that has been mixedwith a binder and compressed by passing through an extruder, typicallyat a temperature at which the binder is in a molten state while thetorrefied wood is compressed. Forming the pellets commonly involveshydraulic extrusion through a fuel pellet die. Of course, other suitablecompression equipment known to those skilled in the art may be used. Thetemperature of the torrefied wood and binder in the extruder is above100° C. but is not allowed to exceed 280° C. to avoid adverse exothermaltorrefication conditions. Preferably the temperature is maintainedbetween 100° C. and 165° C. Another preferred embodiment of the presentinvention uses flax. And yet another embodiment of the present inventionuses whole-tree (white) wood. For use in this invention the white-woodis ground to a size that allows free-flow into the throat of theextruder.

The water-resistant high-energy pellet comprises at least 75% biomassmaterial and preferably at least about 85% biomassmaterial, and theremainder a binder comprising from about 2% to about 25%, preferablyabout 3% to about 15%, by total weight of the pellet. The bindercomprises of a plasticizer in an amount, preferably from 5% to about 20%by weight of the binder, and lignin from about 80% to about 97% byweight of the binder. The optimal ratio of the lignin to plasticizerwill depend upon the characteristics of the biomass material, thelignin, and the plasticizer that are contained within the pellet. Thosechoices typically are made with economics being the primary factor.

Lignin, a by-product of the pulp and paper industry, of sufficiently lowT_(g) will flow and provide protective coverage within the pellet thatis needed for water repellency but may not flow under the conditions ofthe pelletizer which operates at 105° C. and ≧40,000 psi. The individualcharacteristics of the lignin chosen may not be suitable without aplasticizer since its T_(g) may be too high which inhibits its flow atthe temperature of the pelletizer. Adding a plasticizer that itself iswater resistant can provide the requisite T_(g) allowing the binder toflow within the pelletizer yet solidify at ambient temperature. Againthis ratio will be driven by performance and economics once the multiplelarge-volume lignin production facilities come on-line from papermakingoperations and enzymatic conversion of biomass that will have lignin asa byproduct stream.

The plastizers contemplated for use in this invention are totallynatural organics, preferably tall oil pitch, fatty acids, rosin,vegetable oils, animal oils, corn protein, glycerin and the like. Theplastizer serves to plasticize the lignin while providing waterrepellency.

The preferred plastizer contemplated for use in this invention is atotally natural organic, such as glycerin. Glycerin comes from severalsources such as vegetable oil. Glycerin isolated from vegetable oilprocessing provides a ready commercial source of natural plasticizers.

Tall oil pitch is another excellent binder, especially for fire logs.Tall oil pitch is currently commercially used for this application. Thecost of TOP can be relatively high ($400-$600/ton). Tall oil pitch isthe bottoms product from the first distillation column of a refinerythat has tall oil as a feedstock and makes distilled rosins and fattyacids as products. The tall oil pitch may be used, or the “spent” talloil pitch from which the valuable sterol fraction has been removed couldbe used.

Torrefied wood pellets have a bulk density (40-45 lbs/ft³), about twicethat of non-pelletized torrefied wood (16-20 lbs/ft³), so making pelletsdramatically reduces volume and subsequent transportation costs.Torrefied wood pellets have an energy density about the same as coal,12,000 Btu/lb. Size-reduction characteristics of wood pellets—how theybreak down under mechanical forces—are much different than that of coal,which is a problem when large fractions of wood pellets are fed intoexisting pulverizing equipment used by all coal-burning power-generationequipment. However, size-reduction characteristics of torrefied woodpellets are very similar to those of coal. Thus the mechanical handlingcharacteristics and energy density of torrefied wood pellets are verysimilar to coal, making those pellets much more acceptable as a directsubstitute for coal.

As noted, one of the critical characteristics of the high-energy-contenttorrefied wood pellets of this invention is that they are waterrepellant. Another important property is pellet strength and attritionresistance, since dust creates not only worker health and safety issuesbut also dust severely reduces the bulk density of the pellets so that aship loaded with pellets crossing an ocean carries a lower mass loadingcausing the shipping rates per ton to be higher. This densificationeffect is counter-intuitive because the dust particles separate theindividual pellets thereby decreasing the bulk density instead ofpartitioning entirely in the interstitial areas of the bulk pellets.

Many modifications and other embodiments of the invention set forthherein will come to mind to one skilled in the art to which thisinvention pertains having the benefit of the teachings presented in theforegoing descriptions. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

What is claimed is:
 1. A high-energy, water-resistant pellet comprising:(a) at least 75% biomass material; and (b) the remainder of said pelletbeing a two-component binder comprising from about 2% to about 20% bytotal weight of the pellet wherein said binder is a two-component systemcomprising a plasticizer, and the other component is lignin isolatedfrom its source.
 2. The high-energy water resistant pellet according toclaim 1 wherein said high-energy water-resistant pellet comprises atleast 85% biomass material and less than 10% by weight water.
 3. Thehigh-energy water resistant pellet according to claim 1 wherein saidbiomass material is selected from the group consisting of torrefiedwood, whole-tree (white) wood, agricultural waste and flax.
 4. Thehigh-energy water resistant pellet according to claim 1 wherein saidplastizer is selected from the group consisting of tall oil pitch, fattyacids, rosin, vegetable oil, animal oils, corn protein and glycerin. 5.The high-energy water resistant pellet according to claim 1 wherein saidbiomass material is flax.
 6. The high-energy water resistant pelletaccording to claim 1 wherein said plastizer is glycerin.
 7. Thehigh-energy water resistant pellet according to claim 1 wherein saidplasticizer is from about 3% to about 20% of said binder.
 8. Thehigh-energy water resistant pellet according to claim 1 wherein saidlignin is from about 80% to about 97% of said binder.
 9. The high-energywater resistant pellet according to claim 1 wherein high-energy waterresistant pellets have a bulk density 40-45 lbs/ft³.
 10. A high-energy,water-resistant pellet comprising: (a) at least 85% flax wood; and (b)the remainder of said pellet being a two-component binder comprisingfrom about 2% to about 20% by total weight of the pellet, wherein onecomponent of said binder is a plasticizer selected from the groupconsisting of tall oil pitch, fatty acids and rosin, vegetable oil,animal oils, corn protein and glycerin and the other component is ligninin an amount from about 80% to about 97% of said binder.
 11. Thehigh-energy, water-resistant pellet according to claim 10 wherein saidplasticizer is glycerin.
 12. A process for making an energy pellet frombiomass material comprising: (a) preparing a binder by mixing from about2% to about 20% total weight of said binder of lignin and the remainderof said binder being a plasticizer; (b) adding said binder to a biomassmaterial to form a mixture of at least 75% of said biomass material; andthe remainder of said mixture being said binder; and (c) compressingsaid mixture by extruding said mixture at a temperature at which saidbinder is in a molten state to form a pellet.
 13. The process accordingto claim 12 wherein said plastizer is selected from the group consistingof tall oil pitch, fatty acids, rosin, vegetable oil, animal oils, cornprotein and glycerin.